Controlled epitaxial crystal growth by focusing electromagnetic radiation



J J. GROSSMAN CONTROLLED EPITAXIAL CRYSTAL GROWTH BY Aug. 10, 1965FOOUSING ELECTROMAGNETIC RADIATION Filed Jan. 29, 1962.

lw zA/za/e .0 x w/r A a, a 2 m, n 5% I y W 4 AWY W w M K Uit ti spasmscontra tan nrrraxraa crevasse onowrrr av rocnsnso rtrnc'rnostaorsarrcRADlA= This invention relates to controlled epitaxial crystal growth,and more particularly to a method for sustaining the growth processwithin a defined area on a desired substrate.

Vapor growth is a process of growing a solid material from a suitableatmosphere on a substrate. The growth is epitaxial when the materialgrown forms an extension of the crystal structure of the substrate.Vapor growth processes which form such epitaxial layers are popularlycalled epitaxial growth processes and the atmosphere used for such vaporgrowth processes is called an epitaxial growth atmosphere. A variety ofvapor growth processes capable of producing epitaxial layers oncrystalline substrates are known and are generally classified asdispnoportionation processes, reduction processes and thermaldecomposition processes. Such processes have been used to depositsemiconductor materials upon crystalline substrates, as illustrated inU.S. Patents to R. C. Sangster, 2,895,858 and to Christensen et al.2,692,839. By the addition of gases comprising donor or acceptor typeimpurities, the epitaxially deposited layer may be made to be N-type orP-type as desired.

In the production of particular devices, it would be desirable todelinate or control the area on a substrate upon which the growthprocesses proceed. With such control it would be possible to produce avariety of devices having successively deposited layers of differentconductivity, or on different areas of the substrate. Such control hasbeen proposed by the use of jigs, or masks, to cover the portion of thesubstrate where it is desired to inhibit growth, so that upon exposureto the growth atmosphere and conditions, growth will take place only onthe unmasked portions of the substrate. Where successively grown layersare desired, it has been necessary to exchange one mask for anotherbetween successive vapor growth periods. The procedures for the exchangeof masks require manipulations during which the atmosphere surroundingthe substrate is disturbed. Surface oxidation may occur to an extentrequiring subsequent removal of the oxide before the second growthperiod may commence. The present invention provides for successivegrowth periods for producing epitaxial layers in successive areaswithout risk of intermediate oxidation or contamination of previouslydeposited epitaxial layers.

It has been found that the rate of vapor growth of an epitaxial layer onthe substrate material from a gas phase molecule depends upon thefollowing factors:

(a) The partial pressure of the reducing species;

(b) The partial pressure of any oxidizing species;

() The concentration of free radicals;

(d) The activation energy of the reactions;

(e) The temperature on the substrate surface; and

(f) The partial pressure of the substrate species compound which isultimately reduced and deposited as an epitaxial layer.

In the thermal decomposition process the temperature is raisedsufliciently high that the activation energy for the growth process isexceeded by a sufliciently large percentage of the reaction collisionsto produce a useful growth rate. The reaction is generally believed toinvolve free radical mechanics wherein the activation energy for thereaction is the energy required to produce and propagate the freeradicals themselves. In practice substantially uniform heating ofcrystalline substrates results in the producing of uniformly thickepitaxial layers over the entire surface which is exposed to theepitaxial growth atmosphere.

it has been found that a crystalline substrate upon which an epitaxiallayer is to be grown may be held at a temperature just below that whichis sufficient to activate the growth processes and the remaining energy,or driving force, for the reaction may be produced by deliveringelectromagnetic radiation on to the substrate on the area where growthis desired. The rate of growth on the irradiated area of the substratemay be determined by the intensity of and the distribution of theradiation.

The preferred method for delivering and controlling he electromagneticradiation to produce the desired driving force in a defined area is toplace the substrate upon which growth is desired adjacent a window in afurnace chamber through which a suitable growth atmosphere is passed, toposition a light source and an optical system adjacent the window andoutside the chamber to deliver the desired radiation energy through thewindow and and onto the substrate. The optical system may comprise thelight source, an adjacent condensing lens, and a mask between said lensand the window to define the areas on the substrate to be irradiated. Anadditional lensbetween the mask and the window is desirable where theconfiguration of the areas on the substrate is of a definite size,generally much smaller, than the configuration of the apertures in themask through whichthe radiation passes.

As a specific example of controlled area growth by ac tivation of vaporgrowth through electromagnetic radiation, a germanium crystal substrateis deposited in a windowed chamber through which is passed an atmosphereconsisting essentially of GeCl and GeCl as reactants, H as a reducingagent, HCl as an inhibiting agent, helium or argon as an inactivediluent and AsCl as a doping agent. The substrate and the adjacentatmosphere are heated to a temperature within about 50 C. below thethermal reaction temperature at which the vapor growth process willproceed, and an ultraviolet light source is activated to deliver throughthe window suiticient ultraviolet light to provide the activation energyadjacent to the substrate required to sustain the epitaxial growthprocess. The resuling layer of epitaxial material will be N-typegermanium whose conductivity is proportional to the percentage of AsClwhich was present in the vapor growth atmosphere.

For further consideration of what is believed to be novel and myinvention, attention is directed to the drawings in which:

FIG. 1 is a schematic illustration of an apparatus for producing vaporgrowth according to the present invention; and

FIG. 2 is a schematic illustration showing successive layers depositedupon a substrate to produce a semiconductor device.

FIG. 1 is a schematic illustrative showing of apparatus for maintaininga crystalline substrate, such as a slice of P-type germanium on asupport or jig, in a heated chamber at a temperature and in an epitaxialgrowth atmosphere such that the activation energy for the vapor growthprocess is just insufficient for substantial growth. An optical systemis utilized to deliver electromagnetic radiation, in this caseultraviolet light, through a mask and a window and onto the substrate tolocally increase the activation energy adjacent the substrate surfaceand thus to induce localized crystal growth.

As shown in FIG. 1 a furnace chamber 1% whose temperature controlapparatus is not shown in provided with a window 1.1 (which may bequartz) and atoms phere inlet and outlet 12 and 13. Access to thechamber is preferably through a removable end cap 14- for pur: poses ofloading anddischarging jigs and Work thereon. A quartz jig 16 supports agermanium crystal slice 17 the size of which is exaggerated in size forpurposes of illustration. An optical system is illustrated comprising alight source 17 having a light filter Ztl for passing untraviolet light,a condensing lens system 19, a mask 21 and a focusing lens system 22which is preferably designed to focus a reduced image of the aperturesin the mask 21 upon the surface of the substrate 17.

In operation of the apparatus of FIG. 1, a jig 16 with germanium P-typeslice l7 thereon is charged through the end cap 14 into the furnacechamber 1% and positioned adjacent to window 11. The furnace chamber issuitably purged, with hydrogen gas supplied through valve 24, and thefurnace chamber with the work therein is raised to a temperature ofapproximately 709 C. to reduce oxides, then reduced to 450 C, or about50 C below the temperature at which reduction of GeCh takes place. Asthe temperature of the chamber approaches the holding temperature ofabout 450 C, epitaxial growth atmosphere for growth by the hydrogenreduction process is established in the shamber by supplying thereto agas mixture consisting essentially of hydrogen as a carrier and reducinggas, germanium tetrachloride through valve 25 as the germanium sourcegas, and, a minor percentage of arsenic trichloride gas through valve 28of the order of 1% of the amount of germanium tetrachloride supplied.The light source it; is energized and the ultravolet electrogmeticradiation therefrom is focused upon the crystal slice to locallyactivate the crystal growth process. It is believed that the irradiationwith ultraviolet light supplies sutlicient energy at the crystal surfaceto produce free radicals in the atmosphere of H-, GeCl GeCl-, (11-together with other free radicals in lesser concentrations. SutficientAsCl enters into the free radical mechanism to insure the deposit in thegermanium epitaxial layer of arsenic impurity, thus making the layerN-type. A neutral or inactivate diluent gas such as argon or helium maybe supplied to the furnace chamber through valve 24 to help control thediffused spreading of the gas phase reaction and thus localize andcontrol he epitaxial growth in the irradiated surface areas.

When a sufiicient period of time has elapsed for growth of N-typegermanium (arsenic doped) to the desired thickness, as for exampleproducing epitaxial regions 31 and 32 in FIG. 1, the ultraviolet lightsource 13 may be inactivated, the AsCl gas supply may be turned off bythe valve 23, and a boron trichloride gas supply turned on at valve 27.After sufiicient time for purging the atmosphere in the chamber andestablishing the boron trichloride containing atmosphere, the lightsource 18 is turned on and the growth process is continued. If desired anew mask may replace'the mask 21 so that the succeeding areas of growthare somewhat different than the areas as activated through the mask 21.As the vapor growth process proceeds with the boron trichloride impurityin the atmosphere, an epitaxial layer of P-type germanium is grown onthe crystal slice 17. By suitable adjustment of the impurities or dopantmaterials in the atmosphere, and by planned changing of masks, complexstructures may be deposited and grown upon crystalline substrates. Lightand heavy doping of N-or P-type material may be produced by suitableadjustment of the impurity content of the atmosphere, and in those caseswhere there is a tendency to deposit. some material as a noncrystallinelayer, it is preferred to add a small percentage of H01 gas throughvalve 26 as an oxidizing gas to reoxidize non-cpitaxially depositedmaterial. The above process is illustrative of the processes availablefor depositing successive epitaxial layers iii,

of different characteristics without the necessity for intereningmanipulation of the substrate or exposure thereof to undesirableatmosphere or oxidations. The process is generally applicable to thecontrolled epitaxial deposits of silicon semiconductors and tosemiconductors generally known as the Ill-V compounds as illustrated bygallium arsenide, indium phosphide and aluminum antimonide. Although theprocess as above illustrated utilizes chlorine as the halide of thereaction, it will be appreciated that other halogens may be used.Bromine is in some cases preferred and is suitable in both silicon andgermanium technology.

The application of the above described electromagnetic radiationactivated vapor growth process to the pro- .duction of useful devices isillustrated in FIG. 2 wherein the sequential deposit of epitaxial layersis illustrated in the production of a transistor amplifier element withlow resistivity regions thereof for lead attachment. A P-type crystalslice 41 shown in FIG. 2a, is subjected to growth of N-type material, aspreviously described in connection with FIG. -1, to produce to areas 31and 32 as illustrated in FIG. 2b. The mask utilized to produce thelayers in FIG. 2b is then changed to reduce the size of one of the areasand additional N-type material is deposited in areas 31 and 32; as shownin FIG. 20. It is generally preferred to inactivate the light source 18durmg this change over. Theconcentration of AsCl impurity in theatmosphere is next increased and an additional layer of N-I-germaniuinmaterial is deposited through the same mask to produce areas33 and 34 in the configuration of FIG. 2d. The light source 18 is againinterrupted and a new mask inserted while the atmosphere in the furnacechamber 1% is changed :from an AsCl impurity to a BCl impurity and a newarea 35 of P type epitaxial material is deposited on a portion ofpreviously deposited N-type material to produce the transistor emitterconfiguration of FIG. 2e. The mask is again changed and theconcentration of B01 increased to deposit an area 36 of P+ germaniummaterial on the P material just deposited, together with a region 37 ofP+ material deposited upon the original P-type slice 41 producing theconfiguration of the PEG. 2f. The maslois again changed and additionalregion 37 P+ material is deposited to produce the configuration of FIG.2g. The desired structure of the epitaxial layers has now been attainedand the atmosphere within the chamber is now changed with the lightsource 18 turned oif to produce a passivated film 38 on the crystal.This may be done by any conventional passivating procedure such asgrowth of-an oxide film from oxygen or H O plus germanium tetrachloridecontaining atmosphere, or the depoist of an oxide such as'SiO from asuitable decomposition process, as for example the decomposition ofsilanes. The passivating structure is thus produced as shown in FIG. 211which may then be safely removed from the chamber and subsequentlyfurther processed to remove portions of the film 38 as shown in FIG. 21'for lead attachment and encapsulation.

The above illustration of the application of electromagnetic radiationactivated epitaxial layer deposit is an example only of what may beaccomplished,.and variatrons in procedures will become obvious for theconstruction of other layered structures as desired.

Iclaim: i r 1. A method of vapor depositing an epitaxial layer of amaterial on a substrate, which comprises:

establishiu a vapor atmosphere of the material to be deposited adjacentthe substrate, maintaining, the temperature of said substrate less thanthe temperature sufiicient to effect epitaxial vapor deposition;

and focusing electromagnetic radiation on a localized area of thesubstrate in an efiec-tive amount to sustain epitaxial vapor depositionwithin said localized area.

asoaors 2. A method of vapor depositing an epitaxial layer of germaniumon a substrate, which comprises:

establishing an atmosphere adjacent the substrate consisting essentiallyof 63614, GeCl H HCl and reaction products thereof; heating thesubstrate to a temperature less than the temperature sufiicient toelfect epitaxial vapor deposition; and focusing light enery on alocalized area of the surface of the substrate in an efiective amount tosustain epitaxial vapor deposition within said localized area. 3. Amethod of vapor depositing an epitaxial layer of material on a localizedarea of a substrate, which comprises:

placing the substrate in a heating chamber having a window therein withsaid area facing the window; establishing a vapor atmosphere of thematerial to be deposited adjacent said area, heating said substrate to atemperature less than the temperature sufficient to effect epitaxialvapor deposition; and focusing electromagnetic radiation through a maskoutside the chamber, through the window, and onto said localized area ofsaid substrate in an effective amount to sustain epitaxial vapordeposition within said localized area. 4. A method of vapor depositingsuccessive areas of epitaxial material on a substrate, which comprises:

establishing a vapor atmosphere of the material to be deposited adjacentsaid substrate; maintaining the temperature of said substrate less thanthe temperature sufiicient to eiiect epitaxial vapor deposition;focusing light energy on a first localized area of the substrate in aneffective amount to sustain epitaxial vapor deposition within said firstlocalized area; and subsequently focusing light energy on a secondlocalized area of the substrate in an eiiective amount to sustainepitaxial vapor deposition within said second localized area. 5. Amethod of vapor depositing successive areas of a substrate of epitaxialmaterial which comprises:

establishing a first atmosphere of a first material to be depositedadjacent said substrate; maintaining the temperature of said substrateless than the temperature sufficient to effect epitaxial vapordeposition; focusing electromagnetic radiation on a first localized areaof the substrate in an efiective amount to sustain epitaxial vapordeposition within said first localized area;

reducing said radiation to less than said efi'ective amount sufficientto sustain epitaxial vapor deposition;

establishing a second atmosphere of a second material to be depositedadjacent the substrate; and

focusing electromagnetic radiation on a second localized area of thesubstrate in an amount effective to sustain epitaxial vapor depositionwithin said second localized area.

6. The method of claim 5 wherein:

the first atmosphere consists essentially of a predominant quantity of asemiconductor material producing gas and a minor quantity of a firstimpurity producing gas sufiicient to grow an epitaxial layer of a firstconductivity type; and

the second atmosphere consists essentially of a predominant quantity ofsaid semiconductor material producing gas and a minor quantity of asecond impurity producing gas sufiicient to grow an epitaxial layer of asecond conductivity type.

7. The method of claim 6 wherein:

the first atmosphere consists essentially of germanium tetrachloride anda minor quantity of arsenic trichloride; and

the second atmosphere consists essentially of germanium tetrachlorideand a minor quantity of boron trichloridet S. The method of claim 2wherein said light energy is of predominantly ultraviolet light.

References ited by the Examiner UNITED STATES PATENTS 1,364,278 1/21Hochstetter 8824 1,390,445 9/21 Jenkins 8824 2,785,997 3/57 Marvin117107.2 2,916,400 12/59 Homer et al. l17-107.2 3,047,438 7/62 Mar-inacel48-175 FOREIGN PATENTS 1,029,941 5/58 Germany. 1,056,899 5/59 Germany.

DAVID L. R-ECK, Primary Examiner.

1. A METHOD OF VAPOR DEPOSITING AN EPITAXIAL LAYER OF A MATERIAL ON ASUBSTRATE, WHICH COMPRISES: ESTABLISHING A VAPOR ATMOSPHERE OF THEMATERIAL TO BE DEPOSITED ADJACENT THE SUBSTRATE, MAINTAINING THETEMPERATURE OF SAID SUBSTRATE LESS THAN THE TEMPERATURE SUFFICIENT TOEFFECT EPITAXIAL VAPOR DEPOSITION; AND FOCUSING ELECTROMAGNETICRADIATION ON A LOCALIZED AREA OF THE SUBSTRATE IN AN EFFECTIVE AMOUNT TOSUSTAIN EPITAXIAL VAPOR DEPOSITION WITHIN SAID LOCALIZED AREA.